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Monday, August 27, 2012

How much power do you need to fill a venue with sound?

Small venues need small amplifiers. Large venues need racks
and racks of amps. But how do you know how much power is enough?

By David Mellor, Course Director of Audio Masterclass

Working in live sound can encompass venues of all sizes
from a small and intimate bar all the way up to the biggest sports
stadium seating 100,000 or more.

So is there a way to calculate how much power you need to fill the venue with sound?

Firstly,
we need to be a little more precise about this. "Fill the venue with
sound" needs quantifying. One way to do this would be to say that the
system should be capable of a level of 100 dB SPL over the whole seating
area of the venue. Or a map of levels could be produced that allows
some areas to be louder than others.

So now let's consider the variables in the system...

Let's
say that you have 1000 watts of power available. This will be supplied
to loudspeakers that have a certain efficiency rating. If a loudspeaker
can convert 1000 watts of electrical power to 20 watts of sound power,
it is doing pretty well at 2% efficiency. The rest of the energy is
wasted as heat.

So now we have 20 watts of sound power to play
with. All loudspeakers focus their output to a greater or lesser extent.
The more focused the output, the higher the level in the direction of
'throw'.

Now for the difficult part - reflections from the room...

Any
room (in acoustics, 'room' means an enclosed space of any size) holds
and contains sound energy to an extent. A reverberant room will allow
sound energy to bounce back and forth. A well-damped room will absorb
sound energy. The reverberant room will be louder for the same sound
power input because you get the opportunity to hear the same sound
several times as it bounces back and forth.

Plainly, we are
talking about some difficult calculations here. But there is an
alternative... good old-fashioned 'rule of thumb'!

I don't think
you will find a better rule of thumb than that provided by the experts
at Crown Audio who collectively probably have at least as much and
possibly more experience than anyone else in providing amplification for
venues of varying sizes, and for different purposes.

Folk music in a medium-size auditorium, club or house of worship with 150 to 250 seats: 95 to 250 W

Folk music at a small outdoor festival (50 feet from speaker to audience): 250 W

Pop or jazz music in a medium-size auditorium. club or house of worship with 150 to 250 seats: 250 to 750 W

Pop or jazz music in a 2000-seat concert hall: 400 to 1,200 W

Rock music in a medium-size auditorium, club or house of worship with 150 to 250 seats: At least 1,500 W

Rock music at a small outdoor festival (50 feet from speaker to audience): At least 1,000 to 3,000 W

Rock
or heavy metal music in a stadium, arena or amphitheater (100 to 300
feet from speaker to audience): At least 4,000 to 15,000 W

Crown also provide a calculator,
but this does not account for the directional properties of the
loudspeakers, nor for reflections in the room. Still, it makes for a
good starting point. (You could bear in mind that they want to sell you
more amplifiers!)